Why are you casting $c to a float btw? Also, you shouldn't really mix your coding styles (concerning your two different if styles). In my opinion, curly brackets are ALWAYS the best, so i'd suggest using that for all your if statements.
Useful function to retrieve difference in times
The best PHP resource
A good PHP FAQ
PLEASE remember to wrap your code in [PHP] tags.
// Replace this
// With this
// Then check for values/forms. Some IE versions don't send the submit button
Always code as if the guy who ends up maintaining your code will be a violent psychopath who knows where you live.